Is it haram to delay prayer?

Every prayer has a window

Each of the five prayers has a start and an end time. Dhuhr runs until Asr begins, Asr until sunset (though it's disliked to leave it until the sun turns yellow), Maghrib until the red twilight disappears, Isha until the middle of the night according to many scholars, and Fajr until sunrise. As long as you pray inside the window, your prayer is on time. For the basics, see How many times a day do Muslims pray?

Early is best

When the Prophet ﷺ was asked which deed is most beloved to Allah, he said: "Prayer at its proper time" (Sahih al-Bukhari 527). Most scholars understand this as encouraging you to pray early in the window. There are exceptions: delaying Isha a little is recommended if it's not hard on you, and in very hot weather Dhuhr can be delayed until it cools.

Habitually pushing a prayer to the last minute is disliked, though. The Prophet ﷺ described waiting until the sun is about to set and then rushing through Asr as "the prayer of the hypocrite" (Sahih Muslim 622).

Missing the time on purpose

Letting a prayer's time run out with no excuse is a major sin. Allah warns:

Many classical commentators explained "heedless" as people who keep delaying their prayer until its time is gone. If this has happened, repent, make up the prayer, and focus on doing better.

Valid excuses

• Sleeping through it or forgetting: no sin. Pray it as soon as you wake up or remember (Sahih al-Bukhari 597).
• Travel: you may combine Dhuhr with Asr and Maghrib with Isha. See How do I combine Dhuhr and Asr when traveling?
• Illness or real hardship: many scholars, especially Hanbalis, allow combining in genuine need.

Work meetings and busy schedules usually aren't an excuse to miss the window, but they are a reason to plan ahead. Reliable reminders help; see Can I trust prayer time apps?