He was born two years—or perhaps more—before the Hijrah, for his father died in the third year after the Hijrah, leaving four children, the eldest of whom was ʿUmar, followed by Salamah, Zaynab, and Durrah.
It was this ʿUmar who, while still a child, gave his mother in marriage to the Prophet ﷺ (1).
During the Prophet’s own lifetime he grew up, reached puberty, and even married; he once asked about the ruling on a fasting person kissing (2). This alone refutes what Abū ʿUmar recorded in al-Istīʿāb—that he was born in Abyssinia in year two after the Hijrah.
In the second year—indeed, even in the first year—both his parents were already in Medina, and his father had fought at Badr. How, then, could his birth have taken place in Abyssinia in year two? Rather, he must have been born well before that.
After he became the Prophet’s stepson, the Messenger of Allah ﷺ taught him table etiquette, saying: “My boy, come closer, mention Allah’s name, eat with your right hand, and eat from what lies before you” (3).
ʿUmar retained this instruction and many others from the Prophet ﷺ.
(1) Recorded by al-Nasaʾi, 6:81 (Book of Marriage, chapter: The Son Marrying Off His Mother); its chain is sound, as al-Hafiz states in al-Isaba, 4:459. The wording is: When Umm Salama’s waiting period ended, Abu Bakr conveyed the Prophet’s proposal, but she declined. The Messenger of Allah then sent ʿUmar ibn al-Khattab to propose on his behalf. She said, “Tell the Messenger of Allah that I am a jealous woman, that I am a woman with young children, and that none of my guardians is present.” ʿUmar reported this, and the Prophet said, “Return to her and say: As for your saying ‘I am a jealous woman,’ I shall pray that Allah remove your jealousy; as for ‘I am a woman with young children,’ you will look after them; and as for ‘none of my guardians is present,’ none of your guardians, present or absent, objects.” She then told her son, “O ʿUmar, rise and marry the Messenger of Allah,” and he married her to him.
(2) Recorded by Muslim in his Sahih, no. 1108, through ʿAbd Rabbih ibn Saʿid—ʿAbd Allah ibn Kaʿb al-Humayri—ʿUmar ibn Abi Salama: he asked the Messenger of Allah whether a fasting person may kiss his wife. The Prophet said, “Ask this one,” meaning Umm Salama. She told him that the Messenger of Allah did so. He said, “O Messenger of Allah, Allah has forgiven you your past and future sins.” The Prophet replied, “By Allah, I am the most mindful of Allah among you and the most God-fearing.”
(3) Recorded by Malik, 4:934; al-Bukhari, 9:458 (Book of Foods, chapter: Mentioning Allah’s name before eating and eating with the right hand); Muslim, no. 2022 (Book of Drinks, chapter: Etiquette and rulings of food and drink); also by Abu Dawud, no. 3787, and al-Tirmidhi, no. 1858.