Qur'an 5:55: Only Allah is your wali, and His Messenger, and those who believe, those who keep up prayers and pay zakat while they bow [in ruku']. 

__

Based on narrations, Twelver Shi'i and a number of Sunni exegetes have understood this verse to refer to the time when Imam 'Ali (A) gave a ring in charity to someone while he was in ruku'.

(Of course, Sunni exegetes take a different understanding of "wali" and do not draw the conclusion that Imam 'Ali (A) should be caliph, otherwise they would be Shi'i exegetes. Still, the same explanation is presented. There are also some other views on the verse put forward by some Sunni exegetes.)

This view is accepted based on a plurality of narrations expressing this interpretation, rather than a single narration whose chain of narration could be explored. (That is, it comes from many different narrators and was referred to in some various different situations.)

One can argue for the overall truthfulness of this story given that there are a number of narrations on it, and that it was accepted in the non-Shi'i tradition as well. Also, by itself, the verse doesn't make much sense unless it refers to a specific incident, as people do not usually give charity specifically during ruku'.

Some non-Shi'i translators render the verse in English to mean "they give charity AND they are bowing", not WHILE they are bowing, but grammatically, the phrase "wa hum raki'un" seems best to come across as a descriptor of what comes before it rather than a separate clause [that is, to mean WHILE they are bowing]. In any case, it would also not make sense to specifically specify "bowing" as something that people who give charity also do especially since all Muslims are required to give zakat and bow. For that reason, it is particularly helpful to have the narration to understand this last part. [The meaning of God and God's Messenger being our wali is, of course, clear.]

However, since there are multiple narrations on it, it is not necessarily certain which particular narration you are asking about with regard to authenticity. However, if you have a specific narration in mind, please do post again asking about it!

As a suggestion, you can read a number of narrations relating to this verse in Tafsir al-Mizan on this verse (in the section on narrations after the main exegesis); it is available online in Arabic, Farsi, and English (and perhaps other languages also).