Barak Allahu Laka or ‘Alayk: Ward off the Evil Eye by It
Question

Is it prescribed to say, when fearing envy (hasad), “Ma sha Allah la quwwata illa Billah (What Allah willed [has occurred], there is no power except in Allah)”?
Praise be to Allah.How to protect yourself from the evil eye
The correct action according to the Sunnah is for the individual to pray for blessing (barakah) when he sees something that he likes and fears that the  may affect its owner.
The Messenger of Allah (blessings and peace of Allah be upon him) said: “If one of you sees something in himself or his wealth or his brother that he likes, let him pray for blessing (barakah) for it, for the evil eye is real.” (Narrated by Ibn as-Sunni in ‘Amal al-Yawm wa’l-Laylah, p. 168; al-Hakim, 4/216; classed as sahih by al-Albani in al-Kalim at-Tayyib, 243.)
It was narrated that Abu Umamah ibn Sahl ibn Hunayf said: ‘Amir ibn Rabi'ah passed by Sahl ibn Hunayf when he was doing ghusl and said: I have never seen such beautiful skin, not even the skin of young women in seclusion. Straightaway, he [Sahl] fell to the ground. He was brought to the Prophet (blessings and peace of Allah be upon him) and it was said to him: Help Sahl, for he has had a fit (seizure). He said: “Whom do you accuse with regard to him?” They said: ‘Amir ibn Rabi'ah. He said: “Why would any one of you kill his brother? If he sees something in his brother that he likes, then let him pray for blessing (barakah) for him.” Then he called for water, and he told ‘Amir to do wudu, so he washed his face and his arms up to the elbows, his knees and inside his lower garment, then he told him to pour the water over him. (Narrated by Ibn Majah (3509), Ahmad (15550) and Malik (1747)
What to say when you like something and fear the evil eye for it
With regard to what some people say when they like something and fear the evil eye for it, “Ma sha Allah la quwwata illa Billah (What Allah willed [has occurred], there is no strength except in Allah),” there is a hadith that was narrated concerning that. It was narrated by Abu Ya‘la in his Musnad, as is mentioned in al-Matalib al-‘Aliyah (10/348) and Tafsir Ibn Kathir (5/158), from Anas ibn Malik (may Allah be pleased with him), who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Allah, may He be glorified and exalted, never bestows any blessing – such as family, wealth or a child – upon a person and he says “Ma sha Allah la quwwata illa Billah,” but he will never see any troubles in it except death.” And he used to recite this verse: {And why did you, when you entered your garden, not say, 'What Allah willed [has occurred]; there is no power except in Allah} [al-Kahf 18:39].
But the hadith mentioned is da‘if (weak). It was narrated from ‘Abd al-Malik ibn Zarah, whose hadith is da‘if. See: al-Asma wa’s-Sifat by al-Bayhaqi, annotated by ‘Abdullah al-Hashidi (1/417). 
Some of the scholars are of the view that words such as this dhikr are prescribed, if a person sees something that he likes, either for fear of the evil eye and troubles that may befall that thing, or for fear that the owner of that thing may be filled with self-admiration and pride. They understood the meaning of the verse in that way, as is mentioned at the end of the hadith quoted above, which says that he recited that verse.
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said: 
“If a person sees something that he likes, and he fears envy (hasad ) or the evil eye, he should say: Ma sha Allah tabarakAllah, so that the thing looked at will not be affected by the evil eye. Similarly, if a person sees something that he likes in his own wealth, he should say, “Ma sha Allah la quwwata illa Billah,” lest he admire himself and feel proud of this wealth that he admired. So if he says, “Ma sha Allah la quwwata illa Billah,” he will have acknowledged that the source of this blessing is Allah, may He be blessed and exalted.” (Fatawa Nur ‘ala ad-Darb)
He also said:
“The best, if someone fears that he may have affected someone else with the evil eye because he liked or admired him, is for him to say: Tabarak Allahu ‘alayka (may Allah bless it for you), because the Prophet (blessings and peace of Allah be upon him) said to the man who affected his brother with the evil eye: “Why did you not pray for blessing for him?” With regard to saying “Ma sha Allah la quwwata illa Billah,” this should be said by the one who admires his own wealth or property, as the owner of the garden said to his companion: {And why did you, when you entered your garden, not say, 'What Allah willed [has occurred]; there is no power except in Allah} [al-Kahf 18:39].”
In Fatawa al-Lajnah (1/547), it says: 
“As for the word ‘ayn (eye), it comes from the root ‘ana/ya‘in; this refers to affecting someone with the evil eye , for the evil eye is real, as it was narrated in the sahih hadith that the Prophet (blessings and peace of Allah be upon him) said: “The evil eye is real and if anything were to overtake the divine decree, it would be the evil eye. When you are asked to wash yourself (to provide a cure) from the influence of the evil eye, you should wash yourself.” The ruling on it is that it is prohibited, like witchcraft or magic. As for the remedy for the one who affects another with the evil eye , if he sees something that he likes or admires, he should remember Allah and pray for blessing, as mentioned in the hadith: “Why, if you saw something that you liked, did you not pray for blessing?” So he should say, “Ma sha Allah la quwwata illa Billah,” and pray for blessing for that person.” (Fatawa al-Lajnah (1/109)
And Allah knows best.