It is reported in a hadeeth in Muwatta Imaam Maalik that the Messenger (sallal laahu alayhi wasallam) travelled by night on the way back from khaybar and towards the end of the night he stopped for a rest and said to Bilaal (radiyallaahu-anhu), “(Stay awake) to watch out for the morning (prayer) for us…..’’ [Ref 1]

Shaikh Muhammad Baazmool  (may Allaah preserve him) said, “What is the evidence behind the permissibility for a person -before he (or she) sleeps- to watch out for the time by way of (an alarm) clock in order to be notified of (the time of the Salaah)? The evidence is that the Messenger (sallal-laahu-alayhi-wasallam) sought from Bilaal (radiyallaahu-anhu) to guard the time for them, saying,”(Stay awake) to watch out for the morning (prayer) for us”. [Ref 2]

[Ref 1: See complete hadeeth in ‘Muwatta Imaam Maalik- Hadeeth Number 25 under section (The Times of Prayer)]

[Ref 2: Sharh Woqootis Salaah Min Muwatta Al-Imaam Maalik Bin Anas’ page 111. Publisher: Daarul Istiqaamah. 1^st Ed. 1429 (Year 2008). Abridged and paraphrased]