It appears from this hadith that he lengthens the opening supplication; is this valid?

The questioner : May Allah bless you. The established principle is that what is established in voluntary prayer is established in obligatory prayer, unless there is evidence to the contrary. However, regarding some of the wordings of the hadith concerning the night prayer, or when one stands up for prayer at night, does this serve as a specification?

The Shaykh : Yes, this is a specification, because the night prayer has specific characteristics, one of which is lengthening the recitation, the bowing, and the prostration. So the Messenger of Allah (peace and blessings be upon him) would lengthen it. For this reason, nothing was established from him with authentic certainty regarding the opening supplication except This is what is established in the two Sahihs. And the hadith: This is in the Sunan, and the scholars differed regarding its authenticity as a marfu' report.