Abud-Darda' (May Allah be pleased with him) reported: I heard the Messenger of Allah ﷺ saying, "Whenever a Muslim supplicates for his (Muslim) brother in his absence, the angels say: 'May the same be for you too'". Muslim.
This hadith also encourages us to have faith in Allah's mercy and grace; no matter how far away someone may be from us or how difficult a situation they may find themselves in, if we sincerely make dua on their behalf then Allah will answer it through His infinite wisdom and kindness. Furthermore, this hadith serves as an example of how powerful unity among Muslims is; when one person makes dua for another person then all of creation responds positively by praying along with them as well!