All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
In a Hadeeth (narration) on the authority of Az-Zuhri he mentioned that the first wife of the Prophet, sallallaahu ‘alayhi wa sallam, was Khadeejah bint Khuwaylid . She gave birth to Al-Qaasim, At-Taahir, Zaynab, Ruqayyah, Umm Kulthoom and Faatimah all. [Al-Bayhaqi]
In another Hadeeth on the authority of Abu Qataadah it was mentioned that the Prophet, sallallaahu ‘alayhi wa sallam, used to pray while carrying Umaamah, his granddaughter from his daughter Zaynab . [Al-Bukhari and Muslim] Moreover, in a Hadeeth on the authority of Az-Zubayr ibn Bakkaar, may Allaah Have mercy upon him, it was mentioned that when Ruqayyah the wife of ‘Uthmaan died, the Prophet, sallallaahu ‘alayhi wa sallam, allowed him to marry Umm Kulthoom . Then she died and he had no children from her. The Prophet, sallallaahu ‘alayhi wa sallam, told him that if he had ten daughters, he would have made him marry them. [At-Tabaraani]
These Hadeeths prove that Ruqayyah, Zaynab and Umm Kulthoom are all the daughters of the Prophet, sallallaahu ‘alayhi wa sallam.
Allaah Knows best.