Salam. Is anyone aware of a link or a reference to why the burial must happen as soon as possible for Muslims? (It's to give as an answer to some non-Muslims).
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
Wa Alaykum Assalam,
Islam along with other religions, such as Judaism, commands to bury the deceased as soon as the death, without any unneeded delay. This is due to the fact after the soul has departed the body, the body starts to decay and change. The rightful and respectful place for this process to occur is inside the Earth away from the eyes of others.
In addition to this, there have been many narrations from our Prophet Muhammad, peace and blessings be upon him, ordering the Muslims to hasten the burial. If the soul is of a pious person then the soul should be hastened to the enjoyments of the Hereafter; and if the soul is deserving of justice, then it should be hastened to the trial of the Hereafter.
Only Allah knows best
Written by Maulana Shadman Ahmed
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 al-Kāsānī, ’Alā al-Dīn Abū Bakr b. Mas’ūd. Badāi’ al-Ṣanāi’ fī Tartīb al-Sharāi’. Vol. 1. Pg. 299. Dār al-Kutub al-’Ilmmiyyah, 567H.
al-Mullā ’Alī al-Qārī. Mirqāt al-Mafātīh Sharh Mishkāt al-Maṣābīh. Vol. 3. Pg. 1192. Dār al-Fikr, 1014H.