Sayyiduna Ka'b (may Allah be pleased with him) said: Whoever performs ablution well, then prays the Isha prayer and prays four rak'ahs after it, completing their bowing and prostrations, while knowing what he is reciting in them, it will be for him, or he said: these rak'ahs will be for him, equal to Laylat al-Qadr.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4510