Ash-Shaikh Muhammad bin Saalih al-Uthaymeen, rahimahullah, was asked the following question,

رجل يصلي الوتر وأثناء صلاته أذن المؤذن لصلاة الفجر، فهل يتم صلاته‏؟‏

Question:

A man is praying witr and during his prayer, the mua’dhin makes the adhaan for the Fajr Salaat. So should he complete his prayer?

فأجاب فضيلته بقوله‏:‏ نعم، إذا أذن وهو أثناء الوتر فإنه يتم صلاته ولا حرج عليه‏.‏

Answer:

Yes, if the adhaan is made while he is praying witr, then he should complete his prayer and there is no sin upon him.

Reference : Majmoo Fataawa of Ash-Shaykh al-Uthaymeen (#755).

Ash-Shaykh Bin Baaz, rahimahullah, said:

فإذا أذن والإنسان في الوتر كمله والحمد لله ولا بأس ويكفي.

If the adhaan is made while the person is praying witr, he should finish it (i.e. his prayer) walhamdulilah and it will be okay and sufficient

[Reference]

Translated by

AbdulFattaah bin Uthman
Abu Fajr