Allāh 
mentioned musturd seed in the Qurʾān in many different places and contexts.
In surah Luqmaan, Allāh said (when Luqmaan 
was advising his son):
يابني إنها إن تك مثقال حبة من خردل فتكن في صخرة أو في السماوات أو في الأرض يأت بها الله إن الله لطيف خبير
O my son! If it be (anything) equal to the weight of a grain of mustard seed, and though it be in a rock, or in the heavens or in the earth, Allāh will bring it forth. Verily, Allāh is Subtle (in bringing out that grain), Well-Aware (of its place). [Sura Luqman, Ayah 16]
In surah Al-Anbiya, Allāh said
وَنَضَعُ المَوازينَ القِسطَ لِيَومِ القِيامَةِ فَلا تُظلَمُ نَفسٌ شَيئًا ۖ وَإِن كانَ مِثقالَ حَبَّةٍ مِن خَردَلٍ أَتَينا بِها ۗ وَكَفىٰ بِنا حاسِبينَ
And We shall set up balances of justice on the Day of Resurrection, then none will be dealt with unjustly in anything. And if there be the weight of a mustard seed, We will bring it. And Sufficient are We as Reckoners. [Sura Al-Anbiya’, Ayah 47]
These two ayahs go on to show that nothing can escape the reach of Allāh and He is over all things competent.