Is Adding to the Talbiyah Permissible?
Question

Is it permissible to add to the well-known Talbiyah in Hajj and `Umrah?
Praise be to Allah.The Original Talbiyah of the Prophet
The Talbiyah of the Messenger of Allah (blessings and peace of Allah be upon him) was: “Labbayka Allahumma labbayk, labbayka la sharika laka labbayk. Inna Al-hamda wan-ni`mata laka wal-mulk, la sharika lak (Here I am, O Allah, here I am. Here I am, You have no partner, here I am. Verily all praise and blessings are Yours, and all sovereignty, You have no partner).” And he did not say anything more than that. (Narrated by Al-Bukhari, 5915 and Muslim, 1184)
Additional Phrases in Talbiyah by the Prophet
It was also narrated from the Messenger (blessings and peace of Allah be upon him) that he used to say in his Talbiyah “Labbayka ilaha Al-Haqq (Here I am, O God of Truth).” (Narrated by Ahmad, 2/341; classed as authentic by Al-Albani in As-Silsilah As-Sahihah, 2146)
Permissible Additions to the Talbiyah by the Companions
It was narrated from some of the Companions that they added to this format of the Talbiyah.
Nafi` said: `Abdullah in `Umar (may Allah be pleased with him) used to add to it: “Labbayka labbayka wa sa`dayka, wal-khayru bi yadayka, labbayka wal-raghba’u ilayka wal-`aml (Here I am, here I am and blessed by You, and all good is in Your hands; here I am, and aspirations and actions are devoted to You).” (Narrated by Muslim, 1184)
Ibn Abu Shaybah narrated in Al-Musannaf (4/283) that Al-Miswar ibn Makhramah said: The Talbiyah of `Umar was: “Labbayka Allahumma labbayk, labbayka la sharika laka labbayk. Inna Al-hamda wal-ni`mata laka wal-mulk, la sharika lak; Labbayka marghuban aw marhuban, labbayka dha’n-na`ma’i wa’l-fadl il-hasan (Here I am, O Allah, here I am. Here I am, You have no partner, here I am. Verily all praise and blessings are Yours, and all sovereignty, You have no partner; here I am, in fear and hope; here I am, O Bestower of blessings and favours).”
It is proven that the Prophet (blessings and peace of Allah be upon him) approved of these additions that the  Companions (may Allah be pleased with them) made to the Talbiyah and did not object to them, which indicates that they are permissible.
It was narrated that Jabir ibn `Abdullah (may Allah be pleased with him) said, narrating from the Prophet (blessings and peace of Allah be upon him): … , and he proclaimed the Oneness of Allah (Tawhid), saying: “Labbayka Allahumma labbayk, labbayka la sharika laka labbayk. Inna Al-hamda wan-ni`mata laka wal-mulk, la sharika lak (Here I am, O Allah, here I am. Here I am, You have no partner, here I am. Verily all praise and blessings are Yours, and all sovereignty, You have no partner).” The people said this talbiyah that they say nowadays, and the Messenger of Allah (blessings and peace of Allah be upon him) did not object to any of that, but the Messenger of Allah (blessings and peace of Allah be upon him) kept to his own Talbiyah. (Narrated by Muslim, 1218)
Scholarly Opinions on Adding to Talbiyah
Taking all of these Hadiths into consideration, it may be understood that it is preferable for the pilgrim who is performing Hajj or `Umrah to keep to the Talbiyah of the Messenger (blessings and peace of Allah be upon him), but if he adds to it some phrases such as those narrated from some of the Companions (may Allah be pleased with them) or others, that is permissible.
Imam Ash-Shafi`i (may Allah have mercy on him) said – after narrating the wording of the Talbiyah of the Messenger (blessings and peace of Allah be upon him):
This is what I prefer to be the Talbiyah of the pilgrim in Ihram, nothing less and nothing more than that. But if he inserts the words narrated by Abu Hurayrah from the Prophet (blessings and peace of Allah be upon him) – namely the words “Labbayka ilaha Al-Haqq (Here I am, O God of Truth)”– that is close to it in meaning, because it is a Talbiyah, and what is meant by Talbiyah is a response, so he is proclaiming that he is responding to the God of truth by saying Labbayk (Here I am), first and last.
No one should be prohibited from saying something like the words of Ibn `Umar and others, venerating Allah, may He be exalted, and calling upon Him, along with the Talbiyah. But what is preferable in my view is to keep only to the Talbiyah which was narrated from the Prophet (blessings and peace of Allah be upon him) and not add anything to it, apart from that which was narrated from the Prophet (blessings and peace of Allah be upon him), and to venerate Allah, may He be exalted, and call upon Him after stopping reciting the Talbiyah. (End quote from Al-Umm, 2/169-170)
Shaykh Ibn `Uthaymin (may Allah have mercy on him) said:
Question: can we add to the words of the Talbiyah that were narrated by Jabir (may Allah be pleased with him) from the Prophet (blessings and peace of Allah be upon him)?
We say: Yes. Imam Ahmad narrated in Al-Musnad that the Prophet (blessings and peace of Allah be upon him) used to say: “Labbayka ilaha Al-Haqq (Here I am, O God of Truth,” and “God of Truth” is a description of one of His attributes. In other words, one is saying, “Here I am, You are the God of Truth.”
Ibn `Umar (may Allah be pleased with him) used to add the words: “Labbayka wa sa`dayka, wa’l-khayru fi yadayka, wal-raghba’u ilayka wal-`aml (Here I am and blessed by You, and all good is in Your hands, and aspirations and actions are devoted to You).
So if a person adds words such as these, then we hope that there is nothing wrong with that, following the example of `Abdullah ibn `Umar. But it is preferable to keep to that which is proven from the Prophet (blessings and peace of Allah be upon him). (End quote from Ash-Sharh Al-Mumti`, 7/111)
Shaykh Ibn Jibrin (may Allah have mercy on him) said:
Adding to it – i.e., the Prophet’s Talbiyah – is permissible. It was proven that he (blessings and peace of Allah be upon him) used to hear his companions adding to it and he did not change it or object to it. For example:

“Labbayka wa sa`dayka, wal-khayru bi yadayka, wash-sharru laysa ilayka, nahnu `ibaduka Al-wafiduna ilayka, ar-rabighuna fima ladayka (Here I am and blessed by You, and all good is in Your hands, and evil is not to be attributed to You; we are Your slaves who are coming to You, hoping for that which is with You).
“Labbayka wal-raghba’u ilayka wal-`aml (Here I am, and aspirations and actions are devoted to You).”
“Labbayka inna Al-`aysh `aysh Al-akhirah (Here I am; verily the true life is the life of the Hereafter).”
“Labbayka haqqan haqqan ta`abbudan wa riqqan (Here I am, sincerely, sincerely, in worship and servitude (to You).”

All of that is permissible, because it is like a pledge to do these righteous deeds and committing oneself to them. It is also ascribing to Allah, may He be Glorified and Exalted, attributes that befit Him, because all good comes from Him, and all good (deeds) are devoted to Him; He is the Source of all blessings, and evil is not to be attributed to Him. So if a person persists in reciting this, there is the hope that Allah will accept his rituals and worship, and will protect him for the rest of his life. (End quote from Sharh `Umdat Al-Ahkam)
And Allah knows best.