The Essence
While Al-Waahid means the Unique (no one is like Him), Al-Ahad means the Absolute One. He cannot be divided, He is singular in His essence, and He does not consist of parts.
In the Quran
قُلْ هُوَ اللَّهُ أَحَدٌ
"Say, 'He is Allah, [who is] One (Ahad).' (Surah Al-Ikhlas 112:1)"
In Your Duas
يَا أَحَدُ يَا صَمَدُ، نَجِّنِي مِنَ الْهُمُومِ
"O One (Ahad), O Self-Sufficient (Samad), save me from my anxieties."
— General Supplication (Quran 7:180)