Praying for people who are not born yet
He asked for the Anṣār, and then did not stop there.
اللَّهُمَّ اغْفِرْ لِلأَنْصَارِ وَلأَبْنَاءِ الأَنْصَارِ
Allāhumma-ghfir li-l-anṣāri wa li-abnā'i-l-anṣāri wa li-abnā'i abnā'i-l-anṣār.
“O Allah, forgive the Anṣār, and the children of the Anṣār, and the children of the children of the Anṣār.”
Anas was grieving over those killed at al-Ḥarra when he was reminded of this. The prayer had been made for them generations before they were born.
The full narration
Narrated Musa bin `Uqba:`Abdullah bin Al-Fadl told me that Anas bin Malik said, "I was much grieve over those who had been killed in the Battle of Al-Harra. When Zaid bin Arqarr heard of my intense grief (over the killed Ansar), he wrote a letter to me saying that he heard Allah's Messenger (ﷺ) saying, O Allah! Forgive the Ansar and the Ansar children. The subnarrator, Ibn Al-Fadl, is not sure whether the Prophet (ﷺ) also said, And their grand-children." Some of those who were present, asked Anas (about Zaid). He said, "He (Zaid) is the one about whom Allah's Messenger (ﷺ) said, 'He is the one whose sound hearing Allah testified
Source
Ṣaḥīḥ al-Bukhārī · 4906
- Agreed upon as authentic (al-Bukhārī / Muslim)