Narrated from Umm Darda (RA), she said to Safwan (RA), Do you intend to perform Hajj during this year? I said: Yes. She said: Do supplicate Allah for blessings upon us, for Allah's Apostle (ﷺ) used to say: The supplication of a Muslim for his brother at his back (in his absence) is responded so long as he makes a supplica tion for blessings for his brother and the commissioned Angel says: Amen, and says: May it be for you too.

Reference: Muslim: 2733