Times and places when it is mustahabb to say Laa ilaaha ill-Allah
Question

What are the times and places when it is mustahabb to say Laa ilaaha ill-Allah?
Praise be to Allah.The word of Tawheed (i.e., Laa ilaaha ill-Allah – there is no god but Allah), is the greatest word in existence. For it all of creation was created, the Messengers were sent, and the Books were revealed. It is the word of piety, the foundation of the faith and the pillar of belief, and it is the trustworthy handhold which, whoever grasps it will be saved, and whoever dies believing in it will be blessed with a blessing after which he will never be wretched. The virtues of this word and its place in the religion are far greater than can be described or defined.
It was narrated that Jaabir ibn ‘Abdillah (may Allah be pleased with him) said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “The best dhikr is Laa ilaaha ill-Allah (there is no god but Allah), and the best du‘aa’ is al-hamdu Lillah (Praise be to Allah).”
Narrated by at-Tirmidhi (3383) who said: It is hasan ghareeb. Narrated by an-Nasaa’i in as-Sunan al-Kubra (6/208); he included it in a chapter entitled Chapter on the best dhikr and the best du‘aa’. Also narrated by Ibn Hibbaan in his Saheeh (3/126); he included it in a chapter entitled: Statement that praising Allah, may He be glorified and exalted, is one of the best kinds of du‘aa’, and saying Laa ilaaha ill-Allah is one of the best kinds of dhikr.
Classed as hasan by al-Haafiz Ibn Hajar in Nataa’ij al-Afkaar (1/63) and by Shaykh al-Albaani in Saheeh at-Tirmidhi.
Al-Mubaarakfoori (may Allah have mercy on him) said:
Because it is the word of Tawheed, and there is nothing like Tawheed. It is that which differentiates between disbelief and faith. And because it makes one’s mind focus completely on Allah, to the exclusion of all else; and it is more purifying to the soul and cleansing to the heart, and more effective in ridding the nafs of evil and keeping the Shaytaan at bay.
Tuhfat al-Ahwadhi (9/325)
Therefore the successful one among the people of this world is the one who frequently recites these words at all times and in all places, and whose tongue and heart never weary of repeating them, remembering their meanings, and bearing in mind their objectives. These words are among the adhkaar concerning which there is no report to say that it is recommended to recite them  in a specific place or at a specific time; rather the reports speak in general terms so that the Muslim can utter these words in any situation,, when he is busy and when he has free time, when he is going to sleep and when he is awake, when he is at home and when he is travelling, when praying by day or by night, fasting, performing Hajj or ‘umrah, sitting or standing. If he can say them with every breath he takes, then he will be successful and victorious.
Ibn Hajar al-Haytami (may Allah have mercy on him) said:
The best of the adhkaar for which the Lawgiver did not single out a particular situation or time is reciting Qur’an, followed by the words Laa ilaaha ill-Allah, because of the report which says: “The best dhikr is Laa ilaaha ill-Allah.” End quote.
Al-Fataawa al-Hadeethiyyah (p. 109)
Nevertheless, there are several hadiths which encourage reciting this dhikr in specific situations or at specific times. They include the following:
1. After wudu:
“There is no one among you who does wudu and does it well, then says, ‘Ashhadu an laa ilaaha ill-Allah wahdahu laa shareeka lah wa ashhadu anna Muhammadan ‘abduhu wa rasooluhu (I bear witness that there is no god except Allah alone, with no partner or associate, and I bear witness that Muhammad is His slave and Messenger),’ but the eight gates of Paradise will be opened to him and he will enter through whichever one he wishes.’”
Narrated by Muslim (234) from the hadith of ‘Uqbah ibn ‘amir (may Allah be pleased with him).
2. When waking up from sleep during the night:
“Whoever turns over at night and says Laa ilaaha ill-Allah wahdahu laa shareeka lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadeer. Al-hamdu-lillaah wa subhaan-Allah wa laa ilaaha ill-Allah wa Allahu akbar, wa laa hawla wa laa quwwata illa Billaah (There is no god but Allah Alone, with no partner or associate. To Him belongs praise and His is the sovereignty, and He is Able to do all things. Praise be to Allah, glory be to Allah, there is no god but Allah and Allah is Most Great. There is no power and no strength except with Allah), then he says, Allahumma aghfir li (O Allah, forgive me), or he makes du’aa’, his prayer will be granted. If he does wudu and prays, his salaah will be accepted.”
Narrated by al-Bukhari (1154) from the hadith of ‘Ubaadah ibn as-Saamit (may Allah be pleased with him).
3. At the beginning of the day:
““Whoever says “La ilaha ill-Allah wahdahu la shareeka lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shayin qadeer” one hundred times in the day, will have a reward equivalent to that of freeing ten slaves, one hundred hasanahs (good deeds) will be recorded for him, and one hundred sayiahs (bad deeds) will be erased from his record, and it will be protection for him from the shaytan for that day, until evening comes. No one could achieve any better than him except the one who does more than he did.Whoever says Subhaan Allah wa bi hamdih (Praise and glory be to Allah) one hundred times, morning and evening, his sins will be erased even if they are like the foam of the sea.”
Narrated by al-Bukhari (3293) and Muslim (2691) from the hadith of Abu Hurayrah.
4. After saying the salaam at the end of the prayer:
He (the Prophet (blessings and peace of Allah be upon him) used to say following every prayer, after saying the salaam: “Laa ilaaha ill-Allah wahdahu laa shareeka lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadeer. Allahumma laa maani‘ lima a‘tayta wa laa mu‘ti lima mana‘ta wa la yanfa‘ dhaa’l-jadd minka al-jadd  (There is no god but Allah Alone, with no partner or associate, His is the sovereignty and to Him be praise, and He is Able to do all things. O Allah, none can withhold what You give and none can give what You withhold, and no wealth or majesty can benefit anyone for from You is all wealth and majesty).”
Narrated by al-Bukhari (6330) and Muslim (593).
5. At times of distress and hardship
The Prophet (peace and blessings of Allah be upon him) used to say at times of distress: “Laa ilaaha ill-Allah al-‘Azeem al-Haleem, laa ilaaha ill-Allah Rabb al-‘Arsh al-‘azeem, la ilaaha ill-Allah Rabb ul-samawaati wa Rabb ul-ard wa Rabb ul-‘Arsh il-kareem (There is no god but Allah, the All-Powerful, the Forbearing; there is no god but Allah, Lord of the mighty Throne; there is no god but Allah, Lord of heaven, Lord of earth, and Lord of the noble Throne).” Narrated by al-Bukhari (6345) and Muslim (2730), from the hadith of Ibn ‘Abbaas (may Allah be pleased with him).
6. On the day of ‘Arafah.
“The best that I and the Prophets said on the afternoon of (the day of) ‘Arafah is ‘Laa ilaaha ill-Allah wahdahu la shareeka lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadeer (There is no god but Allah alone, with no partner or associate; His is the dominion, to Him be praise, and He has power over all things).”
Narrated by at-Tabaraani in Fadl ‘Ashar Dhi’l-Hijjah; classed as saheeh by al-Albaani in as-Silsilah as-Saheehah (1503).
It is also narrated that it is encouraged to say Laa ilaaha ill-Allah in some hadiths in which there is some weakness, but they were classed as hasan by some scholars. These include the following:
1. It was narrated that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said:
“Renew your faith.” It was said: O Messenger of Allah, how can we renew our faith? He said: “Say a great deal ‘Laa ilaaha ill-Allah.’”
Musnad Ahmad (2/359); classed as saheeh by al-Haakim in al-Mustadrak (4/285); classed as hasan by al-Mundhiri in at-Targheeb wa’t-Tarheeb (2/342); classed as da‘eef by al-Albaani in as-Silsilah ad-Da‘eefah (no. 896).
2. It was narrated that Abu Hurayrah  (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said:
“Frequently recite the testimony that there is no god but Allah before you are prevented from doing so (by death).”
Narrated by Abu Ya‘la in al-Musnad (11/8). Al-Haafiz Ibn Hajar said in al-Futoohaat ar-Rabbaaniyyah (4/110): It is hasan ghareeb. It was classed as hasan by Shaykh al-Albaani in as-Silsilah as-Saheehah (no. 467).
Imam al-Mundhiri collected the hadiths which speak of the virtue of saying Laa ilaaha ill-Allah, in his book at-Targheeb wa’t-Tarheeb (2/265-271).
See also: Fath al-Baari (11/207); an essay entitled Kalimat al-Ikhlaas wa Tahqeeq Ma‘neehaa by al-Haafiz Ibn Rajab al-Hanbali (may Allah have mercy on him); the book Fiqh al-Ad‘iyah wa’l-Adhkaar (1/167-179); an essay entitled Ma‘na Laa ilaaha ill-Allah wa Muqatadaaha wa Athaaruha fi’l-Fard wa’l-Mujtama‘ by Shaykh Saalih al-Fawzaan (may Allah preserve him), published in Majallat al-Buhooth al-Islamiyyah (issue no. 13)
And Allah knows best.