A Believer’s Mention of His Righteous Deeds

The answer is: Allah, Exalted is he, records the reward of a righteous deed that a person performs secretly between himself and his Lord as the reward of secrecy. If the person then discloses it, Allah records for him the reward of publicity. Between the reward of secrecy and the reward of publicity there is a great difference: the reward of secrecy exceeds the reward of publicity by seventyfold, as has been reported in the narrations.
The established practice of the righteous servants of Allah—among the Imams and others—was to conceal acts of worship and to exercise extreme caution in revealing them. It is narrated from al-Hādī (peace be upon him) that one of his companions once became aware of some of his acts of worship. When al-Hādī (peace be upon him) learned that the man had witnessed his worship, his intimate supplication to his Lord, and his weeping throughout the night, he was distressed, and he bound the man firmly to secrecy until death, making him swear to it. The man did not speak of what he had seen of al-Hādī’s (peace be upon him) worship until after his death (peace be upon him).
What we have mentioned—that by disclosing and displaying a righteous deed its reward is reduced from that of secrecy to that of publicity—is the least of the dangerous possibilities. Otherwise, it is possible that the deed may be rendered entirely void by speaking of it.
Accordingly, the prudent course is to conceal one’s deeds and to be cautious about revealing or displaying them, except for a sound purpose of the kind mentioned earlier.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.3