It is narrated from Hazrat Jarir (may Allah be pleased with him) that the Holy Prophet (peace and blessings be upon him) said: When someone flees and joins the enemy and dies there, then he is a disbeliever.
Hadith Referenceمسند احمد / تتمہ مُسْنَدِ الْكُوفِيِّينَ / 19225
Hadith Gradingحکم دارالسلام:حديث صحيح، داود بن يزيد ضعيف لكنه توبع