In عَيْنَ, sākin yāʾ follows fatḥah: it is a līn letter. In connected reading with «الْيَقِينِ», do not lengthen it as if it were a normal madd yāʾ.
If you deliberately pause on «عَيْنَ», its final nūn becomes sākin; a temporary sukūn then follows the līn letter, bringing in the rule of madd līn.
Always ask whether the phrase is being read in connection or at a pause.
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Identifying līn in ʿayna-l-yaqīn
Al-ʿAṣr and At-Takāthur: Applied Ghunnah, Madd, Lām and Waqf
In عَيْنَ, sākin yāʾ follows fatḥah: it is a līn letter. In connected reading with «الْيَقِينِ», do not lengthen it as if it were a normal madd yāʾ.
If you deliberately pause on «عَيْنَ», its final nūn becomes sākin; a temporary sukūn then follows the līn letter, bringing in the rule of madd līn.
Always ask whether the phrase is being read in connection or at a pause.