Zirr b. Hubaish said

I questioned Ubayy b. Ka'b about a statement by his brother [not actual brother] Ibn Mas'ud to the effect that anyone who gets up for prayer every night in the year will hit upon lailat al-qadr, and he replied, “Allah have mercy on him! He meant that people should not content themselves [with that might alone]. He knew that it was in Ramadan among the last ten nights and that it was the twenty-seventh.” He then swore without any reservation that it was the twenty-seventh, and when I asked, “On what ground do you state that, Abul Mundhir? [Ubayy's kunya]” he replied, “By the indication (or sign)* of which Allah's Messenger ﷺ informed us, viz. that the sun rises on that day without rays.” *An alternative, the transmitter not being sure which word was used. *An alternative, the transmitter not being sure which word was used. Muslim transmitted it.