Hawzah News Agency – Imam Ali (AS) said, 'Verily the Prophet of Allah (AS) addressed us one day, saying,

أيُّها الناسُ، إنّهُ قد أقبَلَ إلَيكُم شَهرُ اللَّهِ بالبَرَكَةِ والرّحمَةِ والمَغفِرةِ، شَهرٌ هُو عِندَ اللَّهِ أفضَلُ الشُّهورِ، وأيّامُهُ أفضَلُ الأيّامِ، ولَيالِيهِ أفضَلُ اللَّيالِي، و ساعاتُهُ أفضَلُ الساعاتِ، هُو شَهرٌ دُعِيتُمْ فيهِ إلى‏ ضِيافَةِ اللَّهِ وجُعِلتُم فيهِ مِن أهلِ كَرامَةِ اللَّهِ، أنفاسُكُم فيهِ تَسبيحٌ، ونَومُكُم فيهِ عِبادَةٌ، وعَمَلُكُم فيهِ مَقبولٌ، ودُعاؤكُم فيهِ مُستَجابٌ... فَقُمتُ فقلتُ: يا رسولَ اللَّهِ، ما أفضَلُ الأعمالِ في‏هذا الشَّهرِ؟ فقالَ: يا أباالحسنِ، أفضَلُ الأعمالِ في هذا الشَّهرِ الوَرَعُ عن مَحارِمِ‏اللَّهِ عزّوجلّ.

"O people, verily the month of Allah has come to you with benediction, mercy and forgiveness - a month that is the best of months in the sight of Allah, whose days are the best of days, whose nights are the best of nights, and whose hours are the best of hours. It is a month wherein you have been invited to the banquet of Allah and have been made worthy of Allah's magnanimity. Your breaths during this month are considered glorification [of Allah], and your sleep worship. Your actions in it are accepted and your supplication answered..." Upon hearing this, I stood up and asked, 'O Prophet of Allah, what is the best of deeds to be performed in this month?' He replied, 'O Abu al-Hasan, the best of deeds in this month is to restrain oneself from all that Allah, Mighty and Exalted, has prohibited.'

Source: Amali al-Saduq, p. 84, no. 4