Can you remind me of the supplication said before sleeping in order to wake up for Fajr prayer?

We did not find the supplication (du'a) you are asking about. It appears that if one has internal and external alarms, they will wake up for Fajr prayer and Qiyam (definition) al-Layl (night vigil). Internal alarms include recalling the encouragement for Qiyam al-Layl and Fajr prayer in congregation, such as Allah's saying: (Their sides forsake their beds; they call upon their Lord in fear and aspiration, and from what We have provided them, they spend. No soul knows what has been hidden for them of comfort for eyes as reward for what they used to do), and the Prophet's (peace be upon him) saying: "Whoever prays 'Isha in congregation, it is as if he has stood half the night, and whoever prays Fajr in congregation, it is as if he has prayed the entire night," and "Whoever prays Fajr, he is under the protection of Allah." As for external alarms, they include using alarm clocks, programming one's phone, and advising family members to do so.