Is optional night Prayer an excuse not to have intimate relations with the husband?
Is it permissible for a wife to refuse to be in bed with her husband under the excuse that she observes Qiyam-ul-Layl (optional Prayer at night) or says Tasbih (saying: "Subhan Allah [Glory be to Allah]")?
Are such things Shar`y (Islamically lawful) excuses for her?
No, they are not Shar`y excuses for her; as fulfilling the husband's rights is obligatory while observing Qiyam-ul-Layl and Tasbih 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
Abu Hurayrah (may Allah be pleased with him) who narrated that Allah's Messenger (peace be upon him) said:
"If a husband calls his wife to be in bed with him and she refuses, causing him to get angry with her, the angels will curse her until morning."
(Agreed upon by Al-Bukhari and Muslim)
In another narration, he (peace be upon him) said:
...Until she comes back (to her husband in bed)."
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.