Indeed, those who pledge allegiance to you, [O Muḥammad] - they are actually pledging allegiance to Allāh. The hand1 of Allāh is over their hands.2 So he who breaks his word only breaks it to the detriment of himself. And he who fulfills that which he has promised Allāh - He will give him a great reward.
Translation: Saheeh International (English)
1 See footnote to 2:19.
2 Meaning that He (subḥānahu wa taʿālā) accepted their pledge.
يُبَايِعُونَكَpledge allegiance to youyubāyiʿūnaka
إِنَّمَاonlyinnamā
يُبَايِعُونَthey pledge allegianceyubāyiʿūna
ٱللَّهَ(to) Allahl-laha
يَدُ(The) Handyadu
ٱللَّهِ(of) Allahl-lahi
فَوۡقَ(is) overfawqa
أَيۡدِيهِمۡۚtheir handsaydīhim
فَمَنThen whoeverfaman
نَّكَثَbreaks (his oath)nakatha
فَإِنَّمَاthen onlyfa-innamā
يَنكُثُhe breaksyankuthu
عَلَىٰagainstʿalā
نَفۡسِهِۦۖhimselfnafsihi
وَمَنۡand whoeverwaman
أَوۡفَىٰfulfilsawfā
بِمَاwhatbimā
عَٰهَدَhe has covenantedʿāhada
عَلَيۡهُ(with)ʿalayhu
ٱللَّهَAllahl-laha
فَسَيُؤۡتِيهِsoon He will give himfasayu'tīhi
أَجۡرًاa rewardajran
عَظِيمٗاgreatʿaẓīman
Tafsir
Those who pledge the allegiance of Al-Ridwan to you, O Messenger, to fight the idolaters of Makkah, are only pledging allegiance to Allah, because He is the One Who instructed them to fight the idolaters and He is the One Who will give them recompense. The hand of Allah is over their hands at the time of the pledge and He is aware of them. Nothing of theirs is hidden from them. Whoever breaks his pledge and does not fulfil the promise he made with Allah to assist His religion, then his breaking his pledge and promise is to his own detriment. Allah is not harmed by that. Whoever fulfils his promise to Allah to assist His religion, Allah will grant him a great reward, which is Paradise.