"Tahneek" is regarded as a Sunnah act. Hazrat Abu Musa Al-Ash'ari (may Allah be pleased with him) describes the incident of his son's "tahneek" as follows:
"When my son was born, I brought him to the Messenger of Allah (peace and blessings be upon him). He named him Ibrahim, performed tahneek with a date, prayed for blessings for him, and then returned him to me. This was my firstborn son." (Sahih Bukhari: Hadith No.5467)
To perform "Tahneek", a righteous and pious individual chews a date or another sweet substance, or moistens it with their saliva. They then place it in the baby's mouth by rubbing it on the baby's palate (the roof of the mouth). This is to ensure that the pious individual's saliva reaches the baby's stomach. In this way, the positive influence of the pious individual's good deeds is transferred to the baby.
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دلائل:
صحیح البخاری: (کتاب العقیقة، بَابُ تَسْمِيَةِ الْمَوْلُودِ غَدَاةَ يُولَدُ، لِمَنْ لَمْ يَعُقَّ عَنْهُ، وَتَحْنِيكِهِ رقم الحدیث: 5467، ط: دار طوق النجاة)
حدثني إسحاق بن نصر حدثنا ابو اسامة قال: حدثني بريد عن ابي بردة عن ابي موسى رضي الله عنه قال ولد لي غلام، فاتيت به النبي صلى الله عليه وسلم فسماه إبراهيم، فحنكه بتمرة، ودعا له بالبركة ودفعه إلي، وكان اكبر ولد ابي موسى.
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.