Question 51: Is it permissible for a wife to refuse to be in bed with her husband under the excuse that she observes Qiyaam al-Layl (optional Prayer at night) or says Tasbeeh (saying: “Subhaan Allaah [Glory be to Allaah]”)? Are such things Shar`ee (Islaamically lawful) excuses for her?
Answer: No, they are not Shar`ee excuses for her; as fulfilling the husband’s rights is obligatory while observing Qiyaam-ul-Layl and Tasbeeh are acts of Sunnah (supererogatory acts of worship following the example of the Prophet). The obligatory act of worship prevails over the acts of Sunnah. On the authority of Aboo Hurayrah (may Allaah be pleased with him) who narrated that Allaah’s Messenger (peace be upon him) said... read more here.