Narrated ‘Abdullah bin ‘Umar:
Allah’s Apostle said, “Bilal pronounces the Adhan at night, so keep on eating and drinking (Suhur) till Ibn Um Maktum pronounces the Adhan.”
Narrated ‘Abdullah bin Mas’ud:
The Prophet said, “The Adhan pronounced by Bilal should not stop you from taking Suhur, for he pronounces the Adhan at night, so that the one offering the late night prayer (Tahajjud) from among you might hurry up and the sleeping from among you might wake up. It does not mean that dawn or morning has started.” Then he (the Prophet) pointed with his fingers and raised them up (towards the sky) and then lowered them (towards the earth) like this (Ibn Mas’ud imitated the gesture of the Prophet). Az-Zuhri gestured with his two index fingers which he put on each other and then stretched them to the right and left. These gestures illustrate the way real dawn appears. It spreads left and right horizontally. The dawn that appears in the high sky and lowers down is not the real dawn) .
The Prophet said, “Bilal pronounces the Adhan at night, so eat and drink (Suhur) till Ibn Um Maktum pronounces the Adhan.”
There used to be two Adhaans in Prophet (SAW)’s time. One by Bilal (RA) [before fajr/dawn] and one by Ibn Umm Maktum [for Fajr salah]. The one before fajr (not the prayer, the time i.e. dawn) was pronounced in order to wake people up or inform them of suhoor time. So if anyone wanted to pray tahajjud, they could and if they wanted to eat suhoor, they could. :)
Nowadays, some masajid (mosques) do pronounce Adhaan at tahajjud time. Don’t mix it up with Fajr Adhaan. :)