Fasting: Intention for a Recommended Fast
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1532
[With regard to a recommended (mustaḥabb) fast,] if a person has not done anything that invalidates a fast, then at whatever time of the day he makes the intention to keep a recommended fast – even if there is a short period until maghrib – his fast will be valid (ṣaḥīḥ).