(Your guardian can be only Allah; and His messenger and those who believe…) [5:55].
Said Jabir ibn ‘Abd Allah: “‘Abd Allah ibn Sallam went to the Prophet, Allah bless him and give him peace, and said: ‘O Messenger of Allah, some people from Banu Qurayzah and Banu’l-Nadir have deserted us and swore not to keep our company. And we cannot keep the company of your Companions due to the distance separating us’. He then complained about the bad treatment he received from the Jews, and so this verse was revealed. When the Messenger of Allah, Allah bless him and give him peace, read to him, he said: ‘We accept Allah, His Messenger and the believers as guardians’ ”.
Al-Kalbi related something close to this and added that the end of the verse was revealed about ‘Ali ibn Abi Talib, for he gave his ring to a beggar while he was bowing during prayer.
Abu Bakr al-Tamimi informed us> ‘Abd Allah ibn Muhammad ibn Ja‘far> al-Husayn ibn Muhammad ibn Abi Hurayrah> ‘Abd Allah ibn ‘Abd al-Wahhab> Muhammad ibn al-Aswad> Muhammad ibn Marwan> Muhammad ibn al-Sa’ib> Abu Salih> Ibn ‘Abbas who said: “Accompanied by a group of believers from amongst his people, ‘Abd Allah ibn Sallam came to see the Prophet, Allah bless him and give him peace. They said: ‘O Messenger of Allah, our houses are very far and we have no assembly or a place in which we can gather for chatting. When our people saw that we believed in Allah and His Messenger and confirmed his call, they eschewed us and vowed never to keep our company, speak with us or marry from or into us. This has proved too hard for us’. The Prophet, Allah bless him and give him peace, replied: ‘(Your guardian can be only Allah; and His messenger and those who believe)’, up to the end of the verse. Then the Prophet, Allah bless him and give him peace, went out to the mosque. Some people were standing in prayer while others were bowing. He saw a beggar in the mosque and he asked him: ‘Did you get something from anyone?’ The beggar replied: ‘Yes, I got a ring of gold’. The Prophet asked him: ‘Who gave it to you?’ The beggar said: ‘That person who is standing in prayer’, and he pointed to ‘Ali ibn Abi Talib. The Prophet asked him again: ‘In which position did he give it to you?’ The beggar said: ‘He gave it to me while bowing in prayer’. The Prophet, Allah bless him and give him peace, said ‘Allah is the greatest!’ and then recited (And whoso taketh Allah and His messenger and those who believe for guardian (will know that), lo! the party of Allah, they are the victorious)” [5:56].
Asbab Al-Nuzul by Al-Wahidi , trans. Mokrane Guezzou