Someone who cannot repay someone should make supplication Al-Adab Al-Mufrad 217
Arabic hadith text
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، قَالَ: حَدَّثَنَا حَمَّادُ بْنُ سَلَمَةَ، عَنْ ثَابِتٍ، عَنْ أَنَسٍ، أَنَّ الْمُهَاجِرِينَ قَالُوا: يَا رَسُولَ اللهِ، ذَهَبَ الأَنْصَارُ بِالأَجْرِ كُلِّهِ؟ قَالَ: لاَ، مَا دَعَوْتُمُ اللَّهَ لَهُمْ، وَأَثْنَيْتُمْ عَلَيْهِمْ بِهِ.
Translation
Anas reported that the Muhajirun said, "Messenger of Allah, the Ansar have taken all the reward!" He said, "No, not as long as you make supplication for them and praise them for it."