At the time when Lady Fatimah (PBUH) was pregnant with Lady Zainab (PBUH), the holy Prophet Muhammad (PBUH&HP) was not in Medina. When Lady Zainab (PBUH) was born, Lady Fatimah (PBUH) told Imam Ali (PBUH), “Since my father is on the trip and is not present at this moment, you choose a name for this child.” Imam Ali (PBUH) said, “I will not precede your father (in naming this child); wait until he (the Prophet) returns. Then he will name this child whatever he prefers best.”
Three days later Prophet Muhammad (PBUH&HP) returned from his trip and like all other times, he first went to Lady Fatimah’s (PBUH) house to greet them.
Imam Ali (PBUH) told the Prophet (PBUH&HP) about the newborn baby and how they waited for his return, to name this baby.
The Prophet (PBUH&HP) said, “even though Fatimah’s (PBUH) children are my children, I will wait for a revelation because this matter is with Allah.” At this moment Gabriel descended and said, “O Messenger of Allah, the Lord sends his regards upon you and says, “Name this child ‘Zainab’ because we have written this in a preserved tablet.”
Then the Prophet (PBUH&HP) asked them to bring the newborn baby. He held her close to his chest, kissed her, and named her Zainab. Afterwards he said, “My will to the people, to those who are present and to those who are absent is to keep the respect for this child, for she truly resembles Khadijah.”
(Selection taken from the book “Nasikh al-Tawarikh”, p.44 –section about Lady Zainab (PBUH))
“Congratulations to all on the 5th of Jamadi al-Awwal, the birth anniversary of Lady Zainab (PBUH): symbol of piety and knowledge, and the daughter of Imam Ali and Lady Fatimah (PBUT).”