It is said that salah (Prayer) should be performed at as soon as its time starts,but one place i read it that Isha prayer delaying is sunnah,is it correct?

Alhamdu Lillah

The Prophet SAAW said (Had I not have been worried about you, I would have recommended you delaying Isha prayer). From this Hadith; it is clear that delaying Isha is Sunnah unless one would miss doing it in congregation. So, for women for instance, delaying it till mid-night (not 12:00 am but till the middle of the night between the sunset and the Fajr time of the next day) is acceptable.