[This][1] is the hadith in question:

Narrated Um Habiba:
(daughter of Abu Sufyan) I said, "O Allah's Messenger (ﷺ)! Marry my sister. the daughter of Abu Sufyan." The Prophet (ﷺ) said, "Do you like that?" I replied, "Yes, for even now I am not your only wife and I like that my sister should share the good with me." The Prophet (ﷺ) said, "But that is not lawful for me." I said, We have heard that you want to marry the daughter of Abu Salama." He said, "(You mean) the daughter of Um Salama?" I said, "Yes." He said, "Even if she were not my step-daughter, she would be unlawful for me to marry as she is my foster niece. I and Abu Salama were suckled by Thuwaiba. So you should not present to me your daughters or your sisters (in marriage)." Narrated 'Urwa: Thuwaiba was the freed slave girl of Abu Lahb whom he had manumitted, and then she suckled the Prophet. When Abu Lahb died, one of his relatives saw him in a dream in a very bad state and asked him, "What have you encountered?" Abu Lahb said, "I have not found any rest since I left you, except that I have been given water to drink in this (the space between his thumb and other fingers) and that is because of my manumitting Thuwaiba."

Some scholars have interpreted this hadith to say that Abu Lahab's torment was being relieved for the occasion of the Prophet's birth (which supports the validity of celebrating the birthday of the Prophet), others relate the relief to his action in manumitting (freeing) the slave girl Thuwaiba. [1]: http://sunnah.com/bukhari/67/39

This is the hadith in question:

Narrated Um Habiba:
(daughter of Abu Sufyan) I said, "O Allah's Messenger (ﷺ)! Marry my sister. the daughter of Abu Sufyan." The Prophet (ﷺ) said, "Do you like that?" I replied, "Yes, for even now I am not your only wife and I like that my sister should share the good with me." The Prophet (ﷺ) said, "But that is not lawful for me." I said, We have heard that you want to marry the daughter of Abu Salama." He said, "(You mean) the daughter of Um Salama?" I said, "Yes." He said, "Even if she were not my step-daughter, she would be unlawful for me to marry as she is my foster niece. I and Abu Salama were suckled by Thuwaiba. So you should not present to me your daughters or your sisters (in marriage)." Narrated 'Urwa: Thuwaiba was the freed slave girl of Abu Lahb whom he had manumitted, and then she suckled the Prophet. When Abu Lahb died, one of his relatives saw him in a dream in a very bad state and asked him, "What have you encountered?" Abu Lahb said, "I have not found any rest since I left you, except that I have been given water to drink in this (the space between his thumb and other fingers) and that is because of my manumitting Thuwaiba."

Some scholars have interpreted this hadith to say that Abu Lahab's torment was being relieved for the occasion of the Prophet's birth (which supports the validity of celebrating the birthday of the Prophet), others relate the relief to his action in manumitting (freeing) the slave girl Thuwaiba.